Defense gives both physical damage reduction as well as Crit damage reduction. Physical damage reduction caps at 70%, but one can practically treat the Crit damage reduction from Defense as a stat you won't cap (such as HP, P-Atk, or M-Atk) so more is better. It takes 3 times as much Defense to increase Crit damage reduction by the same amount as normal physical damage reduction.
Overall physical damage reduction is a combination of Defense, physical resistance that corresponds to the type of the incoming attack (Slash, Smash, and Pierce; each caps at 50%), and certificate bonuses that reduce damage taken.*
Magic is unaffected by Defense but is influenced by the specific elemental resistance that corresponds with the type of the incoming magic attack (Fire, Ice, Lightning, Nature, Holy, Dark; each caps at 50%), and certificate bonuses that reduce damage taken.*
Other factors that might influence damage take are passive traits based on race and class, buffs (from players or by entering certain areas), debuffs, Knowledge Point allocation, and gear (including pets) based procs or static effects.
Resistances directly reduce the amount of damage taken by a specific attack. Physical resistance and Defense should be treated differently because alterations to the Defense stat from buffs/debuffs and alterations to Physical resistance are calculated separately. The effectiveness of resistances are usually easier to increase than Defense because resistances come in flat % while Defense is treated as a high number stat where a % of damage reduction is derived based on level. However, Defense and resistances are usually increased in ways that do not interfere with each other.
Achieving the numbers
Fortification and the Defense stat on armor and Shields are used for Defense, though there are two certificates for it as well as enchants that increase Defense. Resistances are gained by crafting armor for Legendary Achievements (Cloth gives elemental resistances, Heavy gives physical resistances, and Light gives Eva bonuses), certificates, trophies and accessories, certificates, gems and enchants, and certain bonuses from armor sets. Most of the conflict between Defense and resistances is for certificate use, but resistances tend to be the more effective certificates to use (not always the case, but is most often true).
Block completely negates damage from an incoming attack. Its activation chance caps at 50% and can be increased with the Str stat as well as direct bonuses to Block % (and one certificate that increases Block rate by a solid 5%). Of the three stats that provide a chance to mitigate or negate damage, this is the most reliable in part because it affects both magic and physical attacks, and because it is hard to work around. Also, Block continues to afford protection even while the player is affected by crowd control statuses such as Stun. Blocking requires use of a Shield on one hand and depletes its durability by 1 for every Block (it is recommended that one use an orange Shield with no durability stat for general tanking use).
Evasion completely dodges an attack so that no ill effects are felt. Its activation chance shows up on the character page to cap at 100%, but this appears to be an indirect indication as to the probability that one will dodge an attack; 100% Eva does not mean one will dodge 100% of incoming attacks. Eva increases with the Agi stat as well as direct bonuses to Eva and four certificates that act as multipliers to Eva bonuses for 8% each (does not add 8% directly to Eva stat but instead sums up + ??? Eva bonuses on gear and gives an 8% bonus to that). In addition, the Accuracy stat neutralizes some of the effect of Eva, so this form of damage negation loses utility against players with decent Acc.** Both magic and physical attacks can be dodged. However, against regular computer generated enemies and against foes with less than amazing Accuracy, Evasion is incredibly effective.
Parry uses a 2-hand weapon and reduces the damage from an incoming physical attack (thus does not affect magic) by 50%. The chance of activation for this caps at 70% and can be increased by the Agi stat (so Agi provides both Eva and Parry) and three certificates that give a high boost by adding 7% directly into the stat.* This form of protection is generally the least effective, though it is more than adequate for farming purposes and the occasional save (Block and Eva are preferred for more serious defense). The downside to using Parry is that it is less powerful than Block, and 2-hand weapons cannot be used with a Shield, so defense is sacrificed for offense (2-hand weapons tend to deal more damage than 1-hand weapons that can be used with Shields). A successful Parry will deduct 1 point of durability from the 2-hand weapon used to perform it.
P-Healing affects how much an incoming heal restores to your HP. Certain orange armor sets increase P-Healing, and there are numerous pieces of gear, enchants, and buffs that will influence P-Healing, but this is an invisible stat and it is unknown in exactly how it is applied. While it is always nice to have, this is not a critical stat since decent healers should be able to restore much lost HP in single heals, especially so when the player's HP is not enormously increased in PvP settings.
As a tank, focus on Block and cap it. This is totally possible on any of the tanking classes (though Dragon Knight is built for Parry using 2-hand weapons and thus forgoes Shield, when equipped with a Shield, it should be able to max as well). Any set when properly fortified will give you a decent amount of defense (aim for 50%). Do all of the Legendary Achievements for armor crafting when you can as well as Mining resources for Str and Agi bonuses. Legendary Achievements for Shields and your weapon of choice depending on the class you use for tanking help, as well as Legendary Achievements for collecting trophies to increase your base HP. HP can also be increased by Class Expertise, an bonus for training many classes (this rewards those who branch out and use the game's class switching mechanic to the fullest). If you want Parry, craft capes, though these consume War Stones and thus require PvP participation. Every tank class has some talents in its talent tree that help its durability. All of the tanks have a talent for HP gain. Templar has Shield defense bonus, Str, and elemental resist. Warrior gets Defense, direct physical damage reduction, and Str. Knight gets Eva, Str, and physical resist. Dragon Knight gets both elemental and physical resist as well as a bonus to Parry.
Use a solid Shield; each comes with its own benefits and weaknesses. An orange crafted Shield has no durability and can have handy effects such as increasing Defense or aggro generation whereas blue Shields tend to have higher base Defense and Block bonuses, but fewer slots for gem additions*** and no special effects (and have randomly generated stats, so it may take some time to find the right one). The same problems can be applied to using orange vs Blue armor, though one may find the blue armor path more effective for Templars than Warriors and Knights can due to their use of magic skills (no orange armors give M-Crit Rate and M-Crit Dmg).
However, every class can use orange armor well if built right. Each new level of orange gear brings 3 different types of armor for each class of armor (Cloth, Heavy, and Light). Two can be crafted, the last requires delivering crafting materials to NPCs in Aven to accumulate Fame with certain groups. Also, Armor Boxes dropped from Legion Elites contain the remaining pieces. The crafted sets are focused on Eva/P-Healing (Wis/Agi) or Crit Rate/Crit Dmg (Str/Lck). The dubbed "Fame sets" focus on Atk Speed/Eva (Agi/Lck). The Agi based sets make it easier to achieve Eva and Parry, but Block must be compensated for. The Fame Atk Speed/Eva set is best for Eva and Parry since it provides the most Agi among the three (the crafted Eva/P-Healing set is close behind it) though a few specific sets provide Parry bonuses. The Crit Rate/Crit Dmg crafted set provides high Str, so at the cost of Eva, it gives higher base Block. No matter which set is used, something else must be compensated for. Oftentimes, the Eva/P-Healing includes a set effect that increases Block Rate, elemental resistances, and/or increased Malice. As for which set to use at level 70, either of the Awakened sets is great. If you don't want to Awaken, one can include any of the six orange/gold armor sets at level 68 (orange) or 70 (gold), though gold armor has the benefit of increased Defense and stats at the cost of having durability.
Offense is also important for tanks since holding aggro (called "Malice" in this game) is an issue. Damage a player deals additionally increases the aggro they generate which is represented by the Malice stat, so dealing high damage makes an enemy more likely to attack you. Tanks often have certain skills that increase the amount of aggro they receive from dealing damage.* There are also several pets that have effects that increase Malice gained from attacks by 5%. Malice is an invisible stat and baffles many players as to how it is actually applied, but we do know how to increase it or lower it to some effect.
High aggro skills
All tank classes have access to Provoke, the most basic taunt skill. It hits one target (has some range leeway) and pulls its attention to the user; for a few seconds afterwards, the enemy will take additional damage from all attacks from the debuff Provoke leaves on it. Knights tend to use Judgement Storm, an AoE skill with moderate cooldown, increases the aggro generated by damage from this skill, and adds a DoT to all enemies hit. Warriors use Group Provoke, a skill with a longer cooldown, but instantly taunts all mobs in range (and the range is enormous) pulling them off allies instantly. Warrior also has Angry Punch which damages a single enemy and create an augmented amount of aggro. Templar has the most trouble holding aggro since Judgement Impact does damage and increases aggro from the hit, but has no other effects (and the cooldown is not short enough to compensate for this). To further exasperate the issue, inexperienced Templars may use Lightning Wave inappropriately; this skill knocks down enemies and incapacitates them for four seconds while increasing the damage they take by 50%. This means the gap between the Templar's dps and the damage output from true dps classes is widened by that much and aggro loss occurs even faster, though this is fine if the 50% damage increase allows the party's dps to finish off mobs before they reach other party members. Otherwise, this is inconvenient and counterproductive.
Every tank has the choice to put in Knowledge Points to increase Malice generated by ?% each. The first 5 levels you raise in any talent costs 1 point each whereas level 6-15 cost 2 points each and 16-20 cost 3 points each. It is up to the individual as to how many points should go into Malice because there are other ways to generate aggro. It should be noted that Knight's Judgement Storm and Warrior's Angry Punch have this Malice talent as a prerequisite to increasing their effects. Also, Warrior's Group Provoke can be powered up with a single Knowledge Point and has another talent that directly reduces damage taken as the prerequisite (so it's good Knowledge Point allocation overall for these talents). Templar's Malice talent caps at 5 levels, but each level increases Malice by 4%, making it definitely worth using (20% Malice increase is where many other tanks would quit putting points in this talent anyway, though this is up to personal preference). It helps also to generate Malice by putting Knowledge Points into commonly used skills so that each does more damage and increases Malice in that way, or investing in Crit Rate for Knight, Warrior, and Dragon Knight (not recommended for Templar since its damage is largely magic based) or M-Crit Rate for Dragon Knight (unfortunately for Templar, it has no M-Crit Rate talent in its tree) for more bursts of damage.
An unorthodox way to increase aggro generation is to use elemental damage. Each elemental damage effects says that attacks will do +??? of [element] damage, and counts as a separate attack on the foe. Declaration of Anger increases aggro generation by ??? points depending on the skill level (you must pay gold to level this skill from your class UI, but it is worth leveling alongside Provoke, your best taunt skill) and is applied to all hits performed by the tank using it. For example, if Declaration of Anger increases your Malice by 100 points and your attack generates 500 Malice, you would gain 600 points of Malice (these numbers are entirely theoretical). If you had an elemental damage effect that inflicts 16 points of Fire damage the opponent, then you would gain 500 Malice from the original attack, 100 Malice from Declaration of Anger acting on said attack, 1 point of Malice from the elemental damage, and another 100 points of Malice from Declaration of Anger on the elemental damage. In this way, Declaration of Anger's effect can be greatly amplified**** since there are numerous ways to stack elemental damage: trophies, waist enchants, 2-hand weapon enchants, armor set effects, weapon effects, accessory effects, and buffs (notably Mage's Element Guardian, which adds Fire, Lightning, and Ice damage all together and is indefinitely maintainable). Another benefit of elemental damage is that, again, since they count as individual hits, they increase the chance that effects from gear that has a % of activation will proc. If a single skill hits an enemy multiple times, elemental damage is applied for each individual hit from that skill. If Double Hit or Triple Hit effects from gear, passive traits, or certificates activate, elemental damage will also be applied to each individual hit.
Quick % recap
Defense (regular damage reduction): 70%
Defense (Crit damage reduction): Presumed 70%, not feasible to cap at upper levels
HP: no cap
Malice: no [known] cap
P-Healing: no [known] cap
* Warrior gives certificate that reduces physical damage taken by first 3% and then 6% in its upgraded form.
Sage gives a certificate that reduces damage from magical Crits by 10% then 20% in its upgraded form.
Dragon Knight provides a certificate that reduces all damage taken by 4% then 8% in its upgraded form.
Dragon Knight also gives a certificate that offers a 20% chance to heal 1000-2000 HP when a Parry is successfully performed, increasing the chance of activation to 40% in its upgraded form.
Warrior gives a certificate that increases Malice generated by 4% then 8% in its upgraded form.
** It should be noted that players often build around Crit Rate and M-Crit Rate to deal high amounts of damage, and the Luck stat raises Acc, Crit Rate, and M-Crit Rate, so they tend to be closely related.
*** Gems, especially when processed by a Human with the correct racial crafting skill, can greatly increases stats such as Lck + Crit Rate, or Lck + Accuracy. For tanking, Agi + [bonus Agi] and Str + Block Rate tend to be good choices for weapons and armor.
**** Provoke (available to all tank classes) and Group Provoke (Warrior skill) inflicts a debuff on the target so that it takes an additional ??? damage depending on the skill level from every incoming attack. When each hits, it will first inflict a smaller hit such as 3 damage on the opponent, and this also triggers the amplifying effect of Declaration of Anger. In addition, the debuff from Provoke and Group Provoke will increase the damage inflicted by all elemental damage. For example, if Provoke increases all damage take by 50 and the elemental damage effect normally deals 13 of Fire and 13 of Ice damage, each will do 63 damage instead. Elemental damage counts as magic damage and is thus reduced by the enemy's resistance.